Urochordata is one of the sub phylum of chordate. Tunicates or Urochordata are all marine and colonial or solitary. Some species lead a sedentary life and others are pelagic. The Ascidian tunicate has formed a wrinkled cylinder or sac with 3-4 inches in length and has a broader base by which the organism is attached to the rocks or other substrates. Large aperture is present at the free end that is more or less terminal position which is the mouth or the inhalant siphon. Atrial or exhalent aperture is present little behind the aperture. Water current flows into the organism by the mouth and passes out through the atrial aperture. Disturbing of the animal makes it to squirts out jets of water through the atrial aperture and this gave the organism the name sea squirt. Anterior end of the animal is made by the oral aperture and posterior end is attached to the substrates and in the dorsal position the atrial aperture is placed.
The body of the animal is enclosed into a tough tunicine, a substance made of cellulose. The underlying ectoderm of the body wall secretes the test as cuticle but the mesoderm cells invade and give rise to the connective tissue. Test is attached in the region of the mouth and artial aperture where the blood vessels enter, others regions of the body wall are not attached and hence a loose covering is formed. The body wall of the animal is called as the mantle which is underlying below the tunic. Mantle consists of a matrix of connective tissue, blood vessels, muscles and nerves are found upon the ectoderm. Mouth and atrial apertures was the region attached to the tunic and produced into short prolongations called as the oral and artial siphons. Sphincter muscles help in closing the apertures.
Mouth leads to the short stomodaeum that is formed by the infolding of the ectoderm. Stomodaeum is followed by the pharynx which occupies the major part of the body from mouth to base. These regions are covered by the atrial or peribranchial cavity...
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